That's a good question. It depends on the test, of course. The Wechsler tests (WISC for kids and WAIS for adults) are fairly reliable and valid. They were tested on large numbers of samples representing as close as possible to the population.

But the test score also reflects the skill of the administrator and other factors related to both people involved. That's why these tests also give you a range of scores your actual score falls into. This is called the confidence interval. It shows you how much error is in the testing, so that rather than one single score, you might actual have a score somewhere within, say 5 points give or take. This interval varies by age and score, as the error varies somewhat. But it's more accurate to look at the confidence interval, and the category, such as Average or High Average, rather than the individual score.

And of course we don't really know what intelligence really is. So it also reflects your performance on THAT test. It may vary on tests of emotional intelligence, practical intelligence, etc.

Hope this helps.

gg